So, I just went halfway "all in" on the game, ordering the bulk of it at once. If we start a campaign now, then when I order the rest in two weeks, if I add it to the Collection, will it appear in our already saved campaign? (Not even mid-game, just at mission/world select or whatever.)
Or is there no reason to start one now if we wish to include the majority for variation?

You can add packs during a campaign, but you can't remove packs during a campaign.

If it's your first campaign with the app, I would consider not adding everything - it will take some time to learn how to handle the imperial activations, so having less variety is not a bad thing. You can add all rebel ally packs though. Adding more expansions later will make a replay of the same campaign a different experience.

+1 what @a1bert said. Be conservative with what you add, starting with only the essentials, and then when you replay the app campaigns later you can add more to change the experience. Especially in the beginning of playing IA you don't really need all the extra stuff as you'll have loads of fun with only the basic stuff at first

Thank you Albert. Just what I needed to know and was hoping so.

Yeah we're going to add in all the packs that replace tokens as we at the expansion to them. And mostly all of the ally packs, and any Villains who make sense. The big boxes, not adding hoth or jabba until those campaigns, probably doing so to the others. Was easily learned, not concerned with that.

Fun fact, just she'd this into be BG Stats app and it's my 99 the game lol (not counting any expansion for any.. so all of IA is 1 game in count)